Systems and methods for polarization mode dispersion mitigation

ABSTRACT

In one exemplary embodiment, a method comprises transmitting an optical signal via the optical line, measuring a relative change in spectral intensity of the optical signal near a clock frequency (or half of that frequency) while varying a polarization of the optical signal between a first state of polarization and a second state of polarization, and using the relative change in spectral intensity of the optical signal to determine and correct the DGD of the optical line. Another method comprises splitting an optical signal traveling through the optical line into a first and second portions having a first and second principal states of polarization of the optical line, converting the first and second portions into a first and second electrical signals, delaying the second electrical signal to create a delayed electrical signal that compensates for a DGD of the optical line, and combining the delayed electrical signal with the first electrical signal to produce a fixed output electrical signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a division of U.S. patent application Ser.No. 11/585,651 entitled “SYSTEMS AND METHODS FOR POLARIZATION MODEDISPERSION MITIGATION,” filed Oct. 24, 2006, which is related toconcurrently filed and commonly assigned U.S. patent application Ser.No. 11/585,659 entitled “OPTICAL TRANSPONDERS WITH REDUCED SENSITIVITYTO POLARIZATION MODE DISPERSION (PMD) AND CHROMATIC DISPERSION (CD),”filed Oct. 24, 2006, the disclosures of which are hereby incorporated byreference herein.

TECHNICAL FIELD

The present invention relates generally to optical systems, and, moreparticularly, to systems and methods for polarization mode dispersion(“PMD”) mitigation.

BACKGROUND OF THE INVENTION

In modern optical networks, signals are often transmitted over hundreds,or even thousands of kilometers. Optical signals traveling overlong-haul and ultra long-haul optical fibers may encounter manydifferent obstacles, including attenuation, chromatic dispersion, andPMD. While attenuation problems have been successfully addressed by theuse of amplifiers and chromatic dispersion by the use of dispersioncompensating fibers, PMD has been much more difficult to handle.

PMD is a phenomenon that occurs when signals with differentpolarizations inside a fiber travel at slightly different speeds, forexample, due to random imperfections and asymmetries of the opticalfiber. This effect causes signal deformation. As a consequence, PMD canmake it very difficult to reliably transmit data at high bit rates. Mostnetworks were built with poor quality fibers in their undergroundinstallations at a time when low bit rates were used and PMD was not yetrecognized as a potential issue. However, now that these structures mustsupport bit rates of 40 Gb/s and higher, PMD presents a significantobstacle to network upgrading.

Generally, the PMD of an optical system cannot be accurately modeled bya single parameter (e.g., its length), but instead it must becharacterized by a series of parameters that represent the entire“history” along the communication line. In practice, however, a few PMDmeasurement and correction systems have been developed. For example,U.S. Pat. No. 5,930,414 to Fishman, et al. and U.S. Pat. No. 6,865,305to Rasmussen, et al. describe an electronic apparatus that measureseye-pattern parameters of a signal—e.g., signal-to-noise-ratio, errorrate, crossing, etc., and thus indirectly determines the PMD of theoptical line. Fishman, et al. further disclose correcting PMD bysplitting the beam signal into two principal states of polarization(“PSPs”), subjecting one of the two PSPs to a relative delay using adelay line, and then recombining the two PSPS. Meanwhile, Rasmussen, etal. teach correcting PMD by using polarization maintaining fibers(“PMFs”).

BRIEF SUMMARY OF THE INVENTION

The present invention relates to systems and methods for polarizationmode dispersion (PMD) mitigators that may be used, for example, tofacilitate the transmission of data across optical networks. It is anobjective of the present invention to provide methods and apparatusesfor measuring and correcting PMD in an optical network. It is anotherobjective of the present invention to provide methods and apparatusesfor reducing first-order PMD (differential group delay or “DGD”) ofsignals traveling through optical lines, including long-haul opticalfibers. Exemplary embodiments of the present invention comprise a PMDmitigator having a PMD measuring module coupled to a control moduleand/or a PMD mitigation device. One of the advantages of the presentinvention is that it provides high performance, low cost, and compactPMD correction devices. Moreover, certain embodiments of the presentinvention may be integrated into an optical transponder and utilize thetransponder's electronic infrastructure, thus reducing design andmanufacturing costs.

In one exemplary embodiment, a method for measuring the DGD of anoptical line comprises transmitting an optical signal having amodulation frequency via the optical line using a polarization scramblerhaving a scrambling frequency, measuring a spectral intensity of asideband of the optical signal at the clock frequency of the opticalsignal (or half of that clock frequency), and using the spectralintensity of the sideband to determine the DGD of the optical line.Another method for measuring the DGD of an optical line comprisestransmitting an optical signal via the optical line, measuring arelative change in spectral intensity of the optical signal at the clockfrequency while varying a polarization of the optical signal between afirst state of polarization and a second state of polarization, andusing the relative change in spectral intensity of the optical signal todetermine the DGD of the optical line.

In another exemplary embodiment, a polarization mode dispersion (PMD)mitigation apparatus comprises an optical detector optically coupled toan optical line, where the optical detector is operable to receive aportion of an optical output signal and convert it into an electricalsignal, a radio-frequency (RF) band pass filter electrically coupled tothe optical detector, where the RF band pass filter is tuned to a clockfrequency and is operable to filter the electrical signal, and an RFdetector electrically coupled to the RF bandpass filter, where the RFdetector is operable to measure an intensity of the filtered electricalsignal. Another polarization mode dispersion (PMD) mitigation apparatuscomprises an optical detector optically coupled to an optical line,where the optical detector is operable to receive a portion of anoptical output signal and convert it into an electrical signal, a clockrecovery unit electrically coupled to the optical detector, where theclock recovery unit is operable to extract a clock signal from theelectrical signal, a combiner electrically coupled to the clock recoveryunit and to the optical detector, where the combiner is operable to mixthe clock signal with the electrical signal, a lowpass filterelectrically coupled to the combiner, where the lowpass filter isoperable to receive an output signal from the combiner and produce afiltered electrical signal, and a radio-frequency (RF) detectorelectrically coupled to the lowpass filter, where the RF detector isoperable to measure an intensity of the filtered electrical signal.

In yet another exemplary embodiment, a polarization mode dispersion(PMD) mitigation device comprises a plurality of optical elements in acascaded configuration and operable to correct PMD of an optical line byoperating on each of two polarization modes of an optical signaltraveling through the optical line, where the optical elements compriseat least one birefringent crystal. Another PMD mitigation devicecomprises a first collimator optically coupled to an optical line, apolarization controller optically coupled to the first collimator, afirst birefringent crystal optically coupled to the polarizationcontroller, a first tunable half-wavelength (λ/2) plate opticallycoupled to the first birefringent crystal, a second birefringent crystaloptically coupled to the first tunable plate, a second tunablehalf-wavelength (λ/2) plate optically coupled to the second birefringentcrystal, a third birefringent crystal optically coupled to the secondtunable plate, and a second collimator optically coupled to the opticalline.

In still another exemplary embodiment, a method for correcting apolarization mode dispersion (PMD) of an optical line comprisessplitting an optical signal traveling through the optical line into afirst portion having a first principal state of polarization and asecond portion having a second principal state of polarization,converting the first and second portions into a first and secondelectrical signals, delaying the second electrical signal to create adelayed electrical signal that compensates for a differential groupdelay (DGD) of the optical line, and mixing the delayed electricalsignal with the first electrical signal to produce a fixed outputelectrical signal. Another polarization mode dispersion (PMD) apparatuscomprises a polarization controller operable to orient an optical signaltraveling through an optical line into two principal states ofpolarization, a polarization beam splitter optically coupled to thepolarization controller and operable to split the optical signal into afirst portion having a first principal state of polarization and asecond portion having a second principal state of polarization, a firstoptical detector optically coupled to the polarization beam splitter andoperable to convert the first portion into a first electrical signal, asecond optical detector optically coupled to the polarization beamsplitter and operable to convert the second portion in to a secondelectrical signal, a tunable electronic delay device electricallycoupled to the second optical detector and operable to compensate for adifferential group delay (DGD) of the optical line, and a combinerelectrically coupled to the first optical detector and the tunableelectronic delay device, where the combiner is operable to produce afixed output electrical signal.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by any person with ordinary skill in the art thatthe conception and specific embodiment disclosed may be readily utilizedas a basis for modifying or designing other structures for carrying outthe same purposes of the present invention. It should also be realizedby any person with ordinary skill in the art that such equivalentconstructions do not depart from the spirit and scope of the inventionas set forth in the appended claims. The novel features which arebelieved to be characteristic of the invention, both as to itsorganization and method of operation, together with further objects andadvantages will be better understood from the following description whenconsidered in connection with the accompanying figures. It is to beexpressly understood, however, that each of the figures is provided forthe purpose of illustration and description only and is not intended asa definition of the limits of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference isnow made to the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a flowchart of a PMD measurement method, according to oneexemplary embodiment of the present invention;

FIG. 2A is a flowchart of a PMD measurement and mitigation method,according to one exemplary embodiment of the present invention;

FIG. 2B is a flowchart of another PMD measurement and mitigation method,according to another exemplary embodiment of the present invention;

FIG. 3 is a block diagram of a PMD mitigator having a filter-basedmeasurement module, according to one exemplary embodiment of the presentinvention;

FIG. 4 is a block diagram of a PMD mitigator having a measurement modulebased on a clock recovery system, according to one exemplary embodimentof the present invention;

FIG. 5 is a diagram of a PMD mitigation device model, according to oneexemplary embodiment of the present invention;

FIG. 6 is a block diagram of a single-stage, free-space PMD mitigationsystem, according to one exemplary embodiment of the present invention;

FIG. 7 is a graph of simulated results obtained with a single-stage PMDmitigation system, according to one exemplary embodiment of the presentinvention;

FIG. 8 is a block diagram of a PMD mitigator in a reflectiveconfiguration, according to yet another exemplary embodiment of thepresent invention;

FIG. 9 is a block diagram of electro-optical hybrid PMD mitigationsystem, according to one exemplary embodiment of the present invention;and

FIG. 10 is a block diagram of a PMD mitigator with a fiber polarizationbeam splitter, according to another exemplary embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, reference is made to the accompanyingdrawings that form a part hereof, and in which is shown by way ofillustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable a person of ordinary skill in the art to practice the invention,and it is to be understood that other embodiments may be utilized andthat structural, logical, optical, and electrical changes may be madewithout departing from the scope of the present invention. The followingdescription is, therefore, not to be taken in a limited sense, and thescope of the present invention is defined by the appended claims.

Polarization mode dispersion (“PMD”) is a polarization-dependentpropagation delay that may be measured in the first order as adifferential group delay (“DGD”) between the fastest and slowestpropagating polarization modes within an optical line. Some exemplaryembodiments of the present invention provide active methods formeasuring and correcting PMD. As such, the PMD of an optical line may bemeasured and used to create a feedback signal in the correction process.In at least one embodiment, the amount of PMD need not be determined inorder for the PMD of the line to be corrected. Turning now to FIG. 1,PMD measurement method 100 is depicted according to one exemplaryembodiment of the present invention. In step 105, a signal modulated ata relatively large frequency (e.g., 5 GHz) is generated and thentransmitted through an optical communication line using a polarizationscrambler having a known scrambling frequency (e.g., 1 MHz). When thereis no DGD in the line, the spectrum of the detected or output signalcomprises a single peak at the modulation frequency. However, when DGDis present, two additional sidebands appear at the two sides of themodulation frequency (i.e., at 5 GHz−1 MHz and 5 GHz+1 MHz).Accordingly, the magnitude of the DGD is manifested in the size of thesidebands.

Still referring to method 100, the spectral intensity of a sideband ofthe optical signal is measured in step 110. In step 115, the spectralintensity of the sideband may be used to determine a DGD of the opticalline. For instance, if Ω is the modulation frequency (e.g., 5 GHz) and ωis the scrambling frequency (e.g., 1 MHz), the intensity measured by thedetector is equal to:I(t)=I ₀{ sin²(ωt/2)cos [Ω(t+Δt)]+cos²(ωt/2)cos [Ω(t−Δt)]},where Δt is the DGD of the line.

Moreover, the spectral intensity of the two sidebands is proportionalto:I(Ω±ω)=I ₀ sin²(ΩΔt)/4.

Therefore, for small DGD, the intensity of the sidebands is proportionalto the square of Δt, or, more specifically,I(Ω±ω)≅I ₀(πΔt/T)²,where T is the period (e.g., 200 ps in the case of a 5 GHz modulationfrequency).

According to method 100, a spectral component may be measured near theclock frequency to avoid having to dedicate a wavelength-divisionmultiplexing (WDM) channel for measurement purposes. The spectrum of anideal PRBS NRZ signal has a shape defined by: sin(πf/f_(c))/(πf/f_(c)).Theoretically, for an ideal pseudo-random binary sequence (PRBS)non-return-to-zero (NRZ) signal, the spectral component at the clockfrequency f_(c)(Ω=2πf_(c)) vanishes. However, for any real signal wherethe length period of the ones is different than the length period of thezeros, a spectral component may be found at f_(c) and it may beexpressed as:I(t)=I ₁(t)+α cos(2πf _(c) t+φ),where I₁(t) is the remainder of the signal (with the spectral envelopesin(πf/f_(c))/(πf/f_(c)), and without a spectral component at f_(c)),and where α and φ are the amplitude and phase at the carrier'sfrequency. Therefore, method 100 may advantageously be used at or nearthe clock frequency where the spectral energy for a PRBS signal isrelatively low.

As previously noted, method 100 requires having a dedicated wavelengthwith given modulation frequency, which means that this dedicatedwavelength cannot be used to carry information. To avoid using adedicated wavelength, a parasitic peak that appears at f_(c) (orf_(c)/2) may be used even if the wavelength carries information.Typically, there is less ambient noise the f_(c) region, thus resultingin more accurate measurements. Moreover, for small DGD values, the clockfrequency is preferred, since the intensity of the signal isproportional to the square of sin(ΩΔt). In other cases, however, it maybe useful to also measure the f_(c)/2 component. For example, innetworks operating at 40 GB/s, the f_(c)/2 component vanishes for DGDvalues around 25 ps. Thus, it may be useful also to measure the halffrequency f_(c)/2, since in this case sin(ΩΔt) vanishes only when theDGD is 50 ps.

Turning now to FIG. 2A, PMD measurement and mitigation method 200A isdepicted according to one exemplary embodiment of the present invention.In step 205A, an optical signal is transmitted via an optical line. Instep 210A, the spectral intensity of the optical signal at or near aclock frequency (or half the clock frequency) is measured. Then, in step215A, the DGD of the optical line is reduced by controlling at least oneoptical component of the optical line so as to increase or maximize thespectral intensity of the optical signal near the clock frequency (orf_(c)/2). It should be noted that a step for measuring the DGD of theline is not required according to this exemplary embodiment.

Referring to FIG. 2B, another PMD measurement and mitigation method 200Ais depicted according to another exemplary embodiment of the presentinvention. In this alternative embodiment, an optical signal istransmitted via an optical line in step 205B. Then, a relative change inthe spectral intensity of the optical signal is measured while thepolarization of the optical signal is varied between first and secondstates of polarization in step 210B. The relative change in spectralintensity of the optical signal may be used in step 215B to determine aDGD of the optical line. In step 220B, optical line's DGD is reduced bycontrolling an optical component of the optical line so as to reduce therelative change in spectral intensity of the optical signal near theclock frequency when the polarization of the optical signal is variedbetween first and second states.

Still referring to method 200, when a signal passes through a mediumwith a first order PMD (i.e., a given DGD Δt), it is split into tworelatively delayed signals represented by:

${{I_{out}(t)} = {{\left( {1 + b} \right){I\left( {t - \frac{\Delta\; t}{2}} \right)}} + {\left( {1 - b} \right){I\left( {t + \frac{\Delta\; t}{2}} \right)}}}},$where b is a coefficient. The spectrum of this signal may be describedas:I _(out)(ω)=I(ω)[(1+b)exp(−iωΔt/2)+(1−b)exp(iωΔt/2)]=2I(ω)[cos(ωΔt/2)−bisin(ωΔt/2)Accordingly, the spectral intensity is:

|I _(out)(ω)|=2I(ω)√{square root over (cos²(ωΔt/2)+sin²(ωΔt/2))}{squareroot over (cos²(ωΔt/2)+sin²(ωΔt/2))}.

And, at the vicinity of the clock frequency, for example:

|I _(out)(ω)|=αδ(ω−2πf _(c))√{square root over(cos²(ωΔt/2)+sin²(ωΔt/2))}{square root over (cos²(ωΔt/2)+sin²(ωΔt/2))}.

where α is a constant and δ(x) is the Dirac delta function. Therefore,when polarization is oriented at one of the principal states ofpolarizations (i.e., b=±1) the spectrum at the clock frequency has amaximum:|I _(out)(ω)|=αδ(ω−2πf _(c)).Meanwhile, when b≠±1 this component gets its maximum value for Δt=0(i.e., for zero DGD). A maximum value of the spectral component (atf_(c) or f_(c)/2) corresponds to a high quality signal—i.e., withoutdeformation caused by DGD.

The relative change in the clock frequency may be expressed as:

${{\eta \equiv \frac{{I\left( {{2\pi\; f_{c}},0} \right)} - {I\left( {{2\pi\; f_{c}},{\Delta\; t}} \right)}}{I\left( {{2\;\pi\; f_{c}},0} \right)}} = {\left( {1 - b^{2}} \right){\sin^{2}\left( {\pi\frac{\Delta\; t}{T}} \right)}}},$where T≡f_(c) ⁻¹ is the bit's period. Therefore, by monitoring η, theDGD Δt may be evaluated. Accordingly, a criterion for system calibrationmay be that η vanish or be reduced or, in other words, thatI_(out)(2πf_(c)) be maximized. While this technique is very sensitive tomeasuring relatively small Δt values, it not useful when Δt=T. Thus,certain embodiments of the present invention measure the two spectralcomponents f_(c) and f_(c)/2. Moreover, as a person of ordinary skill inthe art will readily recognize in light of this disclosure, one of theadvantages of this method is that there is no need to know the absolutevalue of a, because only relative values are relevant.

Referring now to FIG. 3, PMD mitigator 300 having filter-basedmeasurement module 310 is depicted according to one exemplary embodimentof the present invention. PMD mitigator 300 may be used, for example, toimplement one of the PMD measurement and/or calibration methodsdescribed above. In operation, optical input signal 301 passes throughPMD mitigation device 305, which produces optical output signal 302.Optical detector 311 of PMD measurement module 310 is optically coupledto the optical output line of PMD mitigation device 305, and it mayreceive a portion of optical output signal 302 thus converting it intoan electrical signal. In an alternative embodiment, the electricaloutput of an optical detector already existing within the transponder iscoupled to PMD module 310, thus eliminating the need for an additionaldetector. Radio-frequency (RF) bandpass filter 312 is electricallycoupled to optical detector 311 and is operable to filter the electricalsignal received from optical detector 311. In one exemplary embodiment,RF bandpass filter 312 is tuned to the clock frequency of the opticalsystem.

RF detector 313 is electrically coupled to RF bandpass filter 312 and itis operable to measure an intensity of the filtered electrical signalreceived from RF bandpass filter 312. Control module 320 is electricallycoupled to RF detector 313 of PMD measurement module 310, and PMDmitigation device 305 is electrically coupled to control module 320.Accordingly, control module 320 may process an intensity measurementreceived from RF detector 313, and may control at least one opticalcomponent of PMD mitigation device 305 using methods 100 and/or 200 ofFIGS. 1 and 2, for example, to correct or reduce PMD in the opticalline.

FIG. 4 shows PMD mitigator 400 having measurement module 410 based on aclock recovery system according to one exemplary embodiment of thepresent invention. PMD mitigator 400 may also be used to implement oneof the PMD measurement and/or calibration methods described above. Inoperation, optical input signal 401 passes through PMD mitigation device405, which produces optical output signal 402. Optical detector 411 ofPMD measurement module 410 is optically coupled to the optical line, andit is operable to receive a portion of optical output signal 402, thusconverting it into an electrical signal. Clock recovery unit 412 iselectrically coupled to optical detector 411 or to the input electricalsignal and it is operable to extract a clock signal from the electricalsignal. Combiner 413 is electrically coupled to clock recovery unit 412and to optical detector 411, and it is operable to mix the clock signalwith the electrical signal. Lowpass filter 414 is electrically coupledto combiner 413 and it is operable to receive the combiner's outputsignal and produce a filtered electrical signal. In practice, however,because the clock frequency in most 40 Gb/s circuits is 20 GHz ratherthan 40 GHz, an alternative embodiment may comprise a first combiner,low-pass filter, and detector for carrying information about f_(c)(e.g., 20 GHz), and a second combiner, low-pass filter, and detector forcarrying information about 2f_(c) (e.g., 40 GHz). Radio-frequency (RF)detector 415 is electrically coupled to lowpass filter 414 and it isoperable to measure an intensity of the filtered electrical signal. Inone exemplary embodiment, combiner 413 is a high frequency combiner,lowpass filter 414 has a relatively low cutoff frequency (as low as 10kHz), and RF detector 415 is a slow detector.

Control module 420 is electrically coupled to RF detector 415, and PMDmitigation device 405 is electrically coupled to control module 420.Finally, control module 420 is operable to process an intensitymeasurement received from RF detector 415 and to control at least oneoptical component within PMD mitigation device 405 using methods 100and/or 200 of FIGS. 1 and 2 in order to correct or reduce PMD in theoptical line.

Some of the exemplary embodiments described above provide novel andinventive systems and methods for measuring PMD. However, as a person ofordinary skill in the art will readily recognize in light of thisdisclosure, any type of PMD or DGD measuring device or method, otherthan explicitly disclosed herein, may be used along with the PMDmitigation systems and methods depicted below.

Turning now to FIG. 5, PMD mitigation device model 500 is depictedaccording to one exemplary embodiment of the present invention. Undermodel 500, input optical signal 501 is transmitted via an optical linehaving a series of birefringence fibers 503-1-N, where between twosuccessive fibers ones there is a polarization manipulating element502-1-N. Input optical signal 501 reaches PMD mitigation device 550,where the effects of each of components 502-1-N and 503-1-N iseliminated by each components 508-1-N and 507-1-N—i.e., 508-1 cancels502-N, 507-1 cancels 503-N, and so on. Specifically, input opticalsignal 501 enters PMD mitigation device 550 via first collimator 505 andis processed by birefringence elements 508-1-N and 507-1-N, which one byone eliminate the effects of optical line elements 502-1-N and 503-1-N.PMD mitigation device 550 then provides output optical signal 502 viasecond collimator 502.

FIG. 6 shows single-stage, free space PMD mitigation system 600,according to one exemplary embodiment of the present invention. PMDmitigation device 605 of system 600 may be used, for example, toimplement PMD mitigation device(s) 305 and/or 405 of FIGS. 3 and 4according to model 500 depicted in FIG. 5. In one embodiment, mitigationdevice 605 comprises a plurality of free-space optical elements arrangedin a cascaded configuration. In operation, optical input signal 601reaches both PMD mitigation device 605 and PMD measurement module 610.PMD measurement module 610 processes optical input signal 601 andtransmits an electrical signal to control module 611 that is indicativeof the PMD of the optical line. Control module 611 then operates uponPMD mitigation device 605 to reduce or eliminate PMD in the opticalline.

Optical input signal 601 reaches first collimator 611 and passes throughpolarization controller 620. Polarization controller 620 is operable tocompensate for polarization changes that appear at the end of theoptical line. In one embodiment, polarization controller 620 outputs asignal that travels through first birefringent crystal 625, firsttunable λ/2 plate 630, second birefringent crystal 635, second tunableλ/2 plate 640, third birefringent crystal 645, and second collimator 650(all of which are optically coupled to each other in a cascade fashion)thus producing PMD mitigated signal 602. As a person of ordinary skillin the art will readily recognize in light of this disclosure, anarbitrary number of crystals may be used. In some embodiments, crystals625, 635, and 645 may have very large birefringence in order tocompensate for the optical line's birefringence. In other embodimentswhere the polarization properties of the system cannot be described bysuch a “single step” presentation, PMD mitigation device 605 may beconstructed as a multiple-stage device—i.e., Crystal—PolarizationController—Crystal—Polarization Controller, etc. However, if there areno “twists” or other defects in the fiber, it may be possible tocompensate the fiber's birefringence with a single stage systems such asPMD mitigation system 600.

Still referring to FIG. 6, PMD mitigation system 600 is a tunabledevice. In addition, system 600 advantageously operates between the twoprincipal states of polarizations of the line, thus providing arelative, tunable optical delay line. Polarization controller 620 isprovided in front crystals 625, 635, and 645, so that the principalstates of polarization are oriented along the axes of crystals 625, 635,and 645. Crystals 625, 635, and 645 then fix the DGD between the twoprincipal states of polarization of the line.

In one exemplary embodiment, crystals 625, 635, and 645 may be, forexample, Calcite and/or Yttrium Vanadate (YVO4), for which thedifference between the ordinary and extraordinary refraction indices isabout 10%, i.e., |n_(o)−n_(e)|≈0.2. With these types of crystals, thePMD generated by as much as 500 km of an ordinary telecommunicationfiber may be compensated with only about 5 cm of crystal. PMD mitigationdevice 605 may be made even more compact by replacing polarizationcontroller 620 with a small free-space polarization controller, thusleaving collimators 611 and 650 responsible for coupling the input andoutput fibers.

Because the exact birefringence of any given optical line is generallyunknown (in 40 Gb/s networks it can be as large as 25 ps), PMDmitigation device 605 may be made tunable—i.e., its length may bechanged digitally. In the embodiment shown in FIG. 6, birefringencemitigation is performed by three crystals 625, 635, and 645. In otherembodiments, the number of crystals may be changed depending on thedesired precision. Each of crystals 625, 635, and 645 may have adifferent length, and their contributions to the overall mitigation maydepend on the polarization of incident light 601. Therefore, between twosuccessive crystals, tunable half-wave phase retardation (λ/2) plates630 and 640 may be added, thus effectively controlling the sign of thespecific crystal's contribution. That is, each of tunable λ/2 plates 630and 640 may operate in one of two modes: (1) as a 90° polarizationrotator, or (2) as a transparent film—i.e., a full wavelength (λ) plate,keeping the polarization intact. As such, each of plates 630 and 640dictates if the PMD of the crystal that follows it will be added to theoverall device PMD or will be subtracted from it.

If, for example, the time delay between the two polarizationorientations of the crystals are 4 ps, 8 ps and 16 ps respectively (inthe case of YVO4, these delays correspond to crystal lengths of 6 mm, 12mm and 24 mm, respectively), the DGD may be reduced as long as it fallsbetween −28 ps to +28 ps with a resolution of 8 ps. In this example, theworst time-delay of the entire system (with PMD mitigation device 605)is |Δt|≦4 ps. Thus, use of an additional 2 ps crystal may improve theresolution by 4 ps—i.e, |Δt|≦2 ps. Another option is to choose crystalslengths of about 6 mm, 12 mm and 18 mm, in which case PMD correction ofonly ±24 ps is achieved. However, this example allows the system to workin a zero PMD scenario. In one alternative embodiment, PMD mitigationdevice 605 may operate in reflection mode, so that a signal may pass oneor more of crystals 625, 635, and 645 more than once, and thus therequired crystal dimensions may be reduced.

One of the advantages of PMD mitigation device 605 over the prior art isthat it uses birefringence crystals 625, 635, and 645 rather thanoptical fibers and/or or a combination of polarization beam splitterwith delay lines, thus greatly simplifying its design. Another advantageof PMD mitigator 605 over the prior art is that it provides anddiscrete, binary tuning set via tunable plates 630 and 640, as opposedto continuous tuning which is more complex and subject to errors. Thetwo properties make it a relatively small device. As will be readilyrecognized by a person of ordinary skill in the art, PMD mitigator 605may be integrated within an optical transponder, thus resulting in ahigh performance, low cost, and compact device. Moreover, when usedwithin an integrated optical transponder, PMD mitigator 605 may reducefirst order PMD while a multi-level transmitter may mitigate higherorder PMD as described in U.S. patent application Ser. No. 11/585,659,entitled “OPTICAL TRANSPONDERS WITH REDUCED SENSITIVITY TO POLARIZATIONMODE DISPERSION (PMD) AND CHROMATIC DISPERSION (CD).”

FIG. 7 depicts graph 700 of simulated results for a single-stage PMDmitigation system such as the one depicted in FIG. 6, according to oneexemplary embodiment of the present invention. Horizontal axis 705represents time in units of ps, whereas vertical axis 710 representspower in units of μW. Upper curves 701 show the output signal of anetwork with a PMD of about 30 ps. Specifically, signals 701-1 and 701-2show two extreme cases of PMD for the two principal states ofpolarization. Lower curves 702 show the output signal of the PMDmitigation device 605 for the same two principal polarization states,thus demonstrating that PMD mitigation device 605 severely reduces thesystem's PMD.

FIG. 8 shows another PMD mitigator 800 in a reflective configuration,according to another exemplary embodiment of the present invention. Thisparticular embodiment is preferred because it allows PMD mitigator 800to be smaller than the PMD mitigator shown in FIG. 6. Particularly, thisembodiment allows crystals 825, 835, and 845 to have half the size oftheir counterparts in FIG. 6. In this case, input optical signal 801enters DGD mitigator 800 via fiber circulator 855, which may be placedanywhere in the transponder case. Mirror 850 may take the form of ahighly reflecting coating on crystal 845. For example, in the case ofYttrium Vanadate (YVO₄) crystals, where DGD mitigator 800 is designed tomitigate 30±5 ps, the lengths of crystals 825, 835, and 845 may be 11.25mm, 7.5 mm, and 3.75 mm, respectively, which means that the entire DGDmitigator 800 may be made smaller than 40 mm.

Turning now to FIG. 9, electro-optical hybrid PMD mitigation system 900is depicted according to yet another exemplary embodiment of the presentinvention. Hybrid system 900 may be used, for example, as an alternativeto the all-optical PMD mitigation devices 305, 405, 605, and/or 805described in connection with FIGS. 3, 4, 6, and 8. Optical input signal901 is transmitted through polarization beam splitter 915 after passingthrough collimator 905 and polarization controller 910. Polarizationcontroller 910 may orient the two principal states of polarization ofthe uncorrected line and polarization controller 910, so that they matchthe polarization axes of polarization beam splitter 915.

Polarization beam splitter 915 then splits the optical signal into twoportions, where the first portion (which has a first state ofpolarization) is converted into a first electrical signal by firstoptical detector 920, and the second portion (the faster portion, whichrequires delay and has a second state of polarization) is converted intoa second electrical signal by second optical detector 930 after beingreflected by element 925. The second electrical signal is transmittedthrough tunable electronic delay device 935 to correct the opticalline's DGD, and then the first and second electrical signals arerecombined by an analog sum device or combiner 940 to create fixedoutput signal 902. In one alternative embodiment, another tunableelectronic delay device (not shown) processes first electrical signalbefore it reaches analog sum device 940. Further, a Mach-Zehnder (MZ)modulator (not shown) or the like may be used to modulate the output ofa laser (not shown) as a function of fixed output signal 902, thusresulting in an optical signal free from PMD effects of the opticalline.

FIG. 10 is a block diagram of PMD mitigator 1000 with fiber polarizationbeam splitter 1025, according to another exemplary embodiment of thepresent invention. Polarization controller 1030 is fiber-coupled tofiber-polarization beam splitter 1025, which is directly coupled todetectors 1005 and 1010. The output of detector 1005 is transmittedthrough tunable electronic delay device 1015 to correct the opticalline's DGD, and then the first electrical signals are recombined byanalog sum device 1020 to create fixed output signal 1002. One advantageof this configuration is its relative simplicity and ease of assembly.

Although some exemplary embodiments of present invention and theiradvantages have been described above in detail, it should be understoodthat various changes, substitutions and alterations can be made hereinwithout departing from the spirit and scope of the invention as definedby the appended claims. Moreover, the scope of the present invention isnot intended to be limited to the particular embodiments of the process,machine, manufacture, means, methods and steps depicted herein. As aperson of ordinary skill in the art will readily appreciate from thisdisclosure other, processes, machines, manufacture, means, methods, orsteps, presently existing or later to be developed that performsubstantially the same function or achieve substantially the same resultas the corresponding embodiments described herein may be utilizedaccording to the present invention. Accordingly, the appended claims areintended to include within their scope such processes, machines,manufacture, means, methods, or steps.

1. A polarization mode dispersion (PMD) apparatus comprising: apolarization controller; a polarization beam splitter optically coupledto the polarization controller and operable to split an optical signalinto a first portion having a first principal state of polarization ofan optical line and a second portion having a second principal state ofpolarization of the optical line; a first optical detector opticallycoupled to the polarization beam splitter and operable to convert thefirst portion into a first electrical signal; a second optical detectoroptically coupled to the polarization beam splitter and operable toconvert the second portion into a second electrical signal; a tunableelectronic delay device electrically coupled to the second opticaldetector and operable to compensate for a differential group delay (DGD)of the optical line; a combiner electrically coupled to the firstoptical detector and the tunable electronic delay device, where thecombiner is operable to produce a fixed output electrical signal; and alight source configured to output a PMD-compensated optical signal as afunction of the fixed output electrical signal.
 2. The apparatus ofclaim 1, further comprising another tunable electronic delay deviceelectrically coupled to the first optical detector and to the combiner.3. The apparatus of claim 1, wherein the light source is a laser, andthe apparatus further comprises a modulator electrically coupled to thecombiner and operable to modulate the output of a the laser as afunction of the fixed output electrical signal.
 4. The apparatus ofclaim 1, where the polarization beam splitter is a fiber-polarizationbeam splitter.
 5. The apparatus of claim 3 in which the output of thelaser comprises an output optical signal substantially free fromPolarization Mode Dispersion (PMD) effects.
 6. A method comprising:splitting an optical signal using a polarization beam splitter opticallycoupled to a polarization controller into a first portion having a firstprincipal state of polarization of an optical line and a second portionhaving a second principal state of polarization of the optical line;converting the first portion into a first electrical signal using afirst optical detector optically coupled to the polarization beamsplitter; converting the second portion into a second electrical signalusing a second optical detector optically coupled to the polarizationbeam splitter; compensating for a differential group delay (DGD) of theoptical line using a tunable electronic delay device electricallycoupled to the second optical detector; producing a fixed outputelectrical signal using a combiner electrically coupled to the firstoptical detector and the tunable electronic delay device; and producinga PMD-compensated optical signal as a function of the fixed outputelectrical signal.
 7. The method of claim 6, wherein said compensatingfurther utilizes a second tunable electronic delay device electricallycoupled to the first optical detector and to the combiner.
 8. The methodof claim 6, further comprising modulating the output of a laser as afunction of the fixed output electrical signal.
 9. The method of claim 8wherein the output of the laser comprises an output optical signalsubstantially free from Polarization Mode Dispersion (PMD) effects. 10.The method of claim 6, where the polarization beam splitter is afiber-polarization beam splitter.
 11. A method comprising: splitting anoptical signal into a first portion having a first principal state ofpolarization and a second portion having a second principal state ofpolarization; converting the first portion into a first electricalsignal; converting the second portion into a second electrical signal;processing the first and second electrical signals to compensate for adifferential group delay (DGD) of an optical line; combining theprocessed signals to produce a fixed output electrical signal; andproducing an output optical signal as a function of the fixed electricalsignal where the output optical signal compensates for PMD propagationeffects occurring in a signal path prior to the step of splitting theoptical signal.
 12. The method of claim 11, further comprisingmodulating the output of a laser as a function of the fixed outputelectrical signal.
 13. The method of claim 12 wherein the output of thelaser comprises an output optical signal substantially free fromPolarization Mode Dispersion (PMD) effects.
 14. The method of claim 11,wherein the step of splitting an optical signal utilizes afiber-polarization beam splitter.